A detergent-insoluble, actin-containing matrix is associated with the inner surface of the plasma membrane of murine tumor and lymphoid cells. Biochemical and ultrastructural evidence demonstrates that this matrix forms a membrane "skeleton" extending over the entire inner face of the plasma membrane. The matrix consists of seven major protein components. These are being purified and characterized and their interactions are being studied. Experiments will also be done to investigate dynamic aspects of the matrix interactions. These studies of this major component of the cytoplasmic face of the membrane should provide a better understanding of processes such as transmembrane signaling and cell surface protein redistribution, processes crucial to the normal functioning of lymphocytes in the immune system.